1. Field of the Invention
The present invention relates to an image capture apparatus having a function of detecting dust in a focus detecting optical system.
2. Description of the Related Art
Hitherto, in a phase difference type automatic focus (AF) detecting device of an ordinary interchangeable single-lens reflex camera system, two images formed with subject luminous fluxes passing through two exit pupil regions that are different from each other of an interchangeable lens are received with a line sensor pair and subjected to photoelectric conversion. Further, the method of detecting the out-of-focus amount, that is, the defocus amount of the interchangeable lens by obtaining the relative position displacements of image signals which are the outputs of the photoelectric conversion is commonly used.
In that case, the line sensor pair extracts only the luminance distribution of a specified region of the subject space. Accordingly, it is difficult to determine the defocus amount of a subject having no luminance distribution in the region. Therefore, the method of enabling the focus detection for plural subjects by preparing a plurality of line sensor pairs and the focus detecting optical systems corresponding thereto, and extracting the luminance distributions of a plurality of subject regions has been proposed. Further, the method of arranging line sensor pairs in each region so that the line sensor pairs intersect in the vertical direction and the horizontal direction of the photographic screen (hereinafter referred to as the cross phase difference AF) has also been proposed.
More specifically, a cross phase difference type automatic focus detecting optical system will be described with reference to FIG. 13. In FIG. 13, the focus detection is enabled only in the region corresponding to the center of a photographic screen for the sake of simplicity, and line sensors pairing off with each other of the phase difference type AF are arranged in the region so as to intersect in the vertical direction and the horizontal direction of the photographic screen. Therefore, the total number of the line sensor pairs is two. For an automatic focus detecting device which is more commonly used, the method of increasing the detection precision by arranging a plurality of line sensor pairs in the same direction in addition to the cross phase difference type AF, and arranging the pixels of the line sensor pairs so that the pixels are displaced with respect to each other by as much as half a pixel (staggered arrangement) has also been proposed.
The first advantage of the above-described configuration is that the repeatability error in focus detection can be reduced by averaging the results of the focus detection performed with the line sensor pairs. The second advantage of the above-described configuration is that the phase in/out phenomenon can be cancelled (reduced). Namely, the sensitivity of each of pixels included in the line sensor is not uniform so that the sensitivity of the pixel is high at its center and low at its edge. Accordingly, the phase state of a subject image formed on the line sensor is changed for a small movement within a single pixel range and the focus detection result is changed, which denotes the above-described phase in/out phenomenon.
Usually, the total line number of an actual phase difference type automatic focus detecting device is so large that several tens to several hundreds of line sensor pairs are provided.
Returning to FIG. 13, a field mask and a field lens are arranged on the optical axis of the interchangeable lens (capturing lens), so as to be near the scheduled focal plane of the interchangeable lens. Further, two pairs of secondary optical systems are arranged on the optical axis so as to be symmetric and orthogonal with respect to the optical axis. The field lens forms the exit pupil regions of the interchangeable lens on the secondary optical systems so that luminous fluxes passing through two vertically oriented regions are made incident on the vertical line sensor pair and those passing through two horizontally oriented regions are made incident on the horizontal line sensor pair, respectively.
When the focus of the interchangeable lens is achieved in front of the scheduled focal plane in the focus detecting optical system illustrated in FIG. 13, subject images formed on the line sensor pairs are close to each other. When the focus of the interchangeable lens is achieved behind the scheduled focal plane, the subject images are apart from each other. Since a specified function relationship is established between the relative position displacement amount of the subject image and the out-of-focus amount of the interchangeable lens, the defocus amount can be detected by performing an appropriate operation for each of output of the line sensor pairs.
The above-described focus detecting optical system can be ready for various subject patterns, because the line sensor pairs extract vertical and horizontal light amount distributions of the subject. More specifically, the line sensor achieves high focus detection precision for an intersecting contrast. Namely, the horizontal line sensor offers a high contrast for a subject such as a vertical line, and achieves high focus detection precision. On the other hand, the horizontal line sensor offers a low contrast or no contrast for a parallel contrast, that is, a subject such as a horizontal line, and achieves significantly low focus detection precision. Conversely, a subject such as a horizontal line becomes an intersecting contrast for the vertical line sensor, and high focus detection precision is achieved. Providing the cross phase difference AF allows for detecting focuses with stability irrespective of whether the subject is like a vertical line or a horizontal line.
Then, either a vertical defocus amount or a horizontal defocus amount is selected as a definitive defocus amount. Although many methods have been proposed to make the above-described selection, it is common practice to select the amount of a defocus caused in a direction in which the range ring position of the capturing lens becomes closer when the values of the contrast and reliability of the subject image signal are compared to specified threshold values and the values satisfy the specified threshold values.
In the past, airborne dust such as motes and particles was adhered to a main mirror or a sub mirror arranged on the optical path of an AF sensor or a focus detecting optical system (hereinafter referred to as the focus detecting system dust) at the time of mounting and demounting an interchangeable lens, and the focus detection precision was significantly decreased.
More specifically, the focus detecting system dust causes the image signal output of a given line sensor pair to be decreased as illustrated in FIGS. 15A and 15B where the vertical axis indicates the image output of the line sensor pair and the horizontal axis indicates the pixel position of the line sensor pair. When the focus detecting system dust is reflected in an image attained by the focus detecting optical system, the amount of a defocus caused in a direction in which the range ring position of the capturing lens becomes closer is detected even though a subject offers no contrast.
Accordingly, Japanese Patent No. 2952215 pays attention to the fact that a false defocus amount detected due to the focus detecting system dust is repeatedly detected no matter how much the focusing lens of the capturing lens is driven. Japanese Patent No. 2952215 discloses the technology for determining the above-described state to be the adhesion of the focus detecting system dust and prohibiting the corresponding line sensor from performing the focus detection. However, in the known technology disclosed in Japanese Patent No. 2952215, when the camera detects the focus detecting system dust and removes the corresponding line sensor from the list of focus detection targets, the automatic focus adjustment can be performed with the other line sensors even though it is difficult for the camera to offer its original performance. Accordingly, the user recognizes the adhesion of the focus detecting system dust with difficulty.
In addition, even though the user is notified of the detection of the focus detecting system dust with a warning or the like, the user should have knowledge of the camera and special-purpose tools to remove the focus detecting system dust. Therefore, it is difficult for the user to remove the dust. Further, it is troublesome for the user to be warned of the detection every time the automatic focus adjustment is executed during the capturing operations even though it is difficult for the user to remove the dust.
Finally, a worker of a repair center has to apply uniform light with no contrast to the focus detecting system to confirm whether or not an image signal output from a specified pixel is decreased for each of the line sensors, so as to find the adhesion of the focus detecting system dust.